High-pressure tank with permeation barrier

ABSTRACT

The present invention relates to a high-pressure tank made from fiber-reinforced plastic for, in particular, gaseous media, wherein, on its inner wall, the tank is equipped completely or partially with a substantially permeation-tight foil made of metal, wherein the metal has a high elastic range and a low thermal expansion coefficient, and the foil has a thickness of ≦0.5 mm. The invention also relates to a method for manufacturing tanks of this type.

This application is a continuation of U.S. application Ser. No.13/701,577,filed Mar. 12, 2013, which is a 35 U.S.C. 371 National PhaseEntry Application from PCT/EP2011/01607, filed Mar. 30, 2011, whichclaims the benefit of German Patent Application No. 10 2010 022 342.5filed on Jun. 1, 2010, the disclosure of which is incorporated herein inits entirety by reference.

The present invention relates to high-pressure tanks for receiving andholding media such as helium (He), neon (Ne), argon (Ar), krypton (Kr),xenon (Xe) as well as air, oxygen (O₂) and possibly also hydrogen (H₂).In addition to gaseous media, the tanks should also be able to receiveand hold liquids or media that may be dropped. The high-pressure tanksare composed of fiber-reinforced plastic (fiber composite) and feature apermeation barrier. The invention also relates to a process formanufacturing such high-pressure tanks equipped with a permeationbarrier.

High-pressure tanks today are nearly predominantly built offiber-reinforced plastic. Fiber-reinforced plastics are able toincorporate significant characteristics best suited for high-pressuretanks such as light weight as well as mechanical and thermal stabilityor strength.

However, tanks made of fiber-reinforced plastics are not per senecessarily permeation-tight if they are not to be negatively influencedas to their thickness and thus their weight.

It is already known to equip tanks made of fiber-reinforced plasticswith a permeation barrier. As a general rule, plastic liners are used,for example thermoplastic liner materials. However, plastic liners tendto be susceptible to local leaks caused by elongation at defect spotsand joints. Their leak rate should be approximately three orders ofmagnitude lesser than that of metal liners, for example. They aretherefore not suited for long-term missions with helium, for example inspace travel.

U.S. Pat. No. 5,798,156 describes a light-weight compressed gas tankwith an inside liner made of polymeric material. The liner is made of athin polymeric layer which itself may or may not be coated with a thinlayer of material with low permeability, such as silver, gold oraluminum. The polymeric liner is described in context with pressuretanks for compressed gases made from graphite epoxy composite. Thedensity ρ (ρ=m/V) in case of uncoated polymeric liners is e.g. 0.925g/cm³ and 1.965 g/cm³, for LDPE (low density polyethylene) with a goldcoating as opposed thereto it is 19.29 g/cm³.

Metal liners are also used as permeation barriers. However, those metalliners, which might hitherto be used as permeation barriers, are sothick that weight might pose a problem. This applies particularly if thehigh-pressure tank is supposed to be used in aerospace application.

Metal liners are, for example, known from DE 28 15 772 A1, which, amongother things, suggests the use of nickel-titanium alloys with a titaniumcontent of 44 to 47%. The liner's wall thickness is stated as being 0.5mm.

U.S. Pat. No. 3,312,575 suggests ductile nickel as liner material with awall thickness of 0.76 mm.

The intent of the present invention is to show alternatives to thisstate of the art which are technically and/or commercially advantageousand allow a wide range of potential applications for the high-pressuretanks.

In other words, the intent is to make available high-pressure tanks witha high degree of thermal and mechanical resilience, light weight and areliably high degree of permeation resistance also in the presence ofcryogenic temperatures and up to +100° C.

It has been determined that the afore stated disadvantages may beomitted by choosing a mode of construction and/or material which takesthe following criteria in consideration:

1. material with a very high elastic range (ε=R_(p0,2)/E) according toHooke's law, with R_(p0,2) being the yield point and E the elasticmodulus;

2. material with an as small as possible thermal expansion coefficientα;

3. foil thickness;

4. joining by material engagement for example with overlap, electronicwelding and pressure welding connection, with closing the liner beingthe major part of development.

Joining by material engagement may also be accomplished by way ofbonding or other known methods.

In any case the objective is to reduce the liner's mass as far aspossible. This may be achieved by selecting the right material for theliner and by minimizing liner thickness.

As a consequence, the present invention suggests using metal foils.

The present invention thus in its first aspect relates to ahigh-pressure tank made of fiber-reinforced plastic for particularlygaseous media, wherein the tank at its inner wall is equipped with asubstantially permeation-tight metal foil having a great elastic range,a low thermal expansion coefficient and a thickness of ≦0.5 mm.

Preferred embodiments are given in the subclaims.

Thus, the thickness of the permeation-tight metal foil is preferably inthe μm-range. This may result in a distinct and advantageous reductionin weight while at the same time providing for reliablepermeation-tightness for gases.

The selected metal foil is preferably also impermeable for small-atomand small-molecular gases such as helium (He) and nitrogen (N₂), so thatthe tank may be used for storage and transport of these gases as isrequired in aerospace applications.

The metal foil for the particular fields of application shouldpreferably be based upon a metal alloy resistant to compressed hydrogen,which assures that the liner quality also under these conditions willnot change over time, in particular as relates to non-permeability.

Particularly preferred metal foils are based upon titanium alloys,preferably Ti-15-3-3-3. With these alloys a suitable rolling can becarried out at room temperature.

As pertains to the above aspects 1 and 2, the following must be noted:

Aluminum foils are considered as liners or permeation barriers. Eventhough they have a rather unfavorable thermal expansion coefficient,they are indeed suitable for application fields in which thiscircumstance is not significant. As a general rule, they have asufficiently wide elastic range ε (see attached table).

However, it appeared that high-strength titanium alloys are best suitedin this respect as long as the use of hydrogen or cryogenic temperaturesare not foreseen as operation conditions. High-strength special nickelalloys owing to their at the same time slight elastic modulus meet therequirements for a high elastic range.

Metal foils with a low thermal expansion coefficient a are to beexpected among titanium and its alloys, iron alloys with cubic bodycentered grid structure, some nickel basis alloys as well as alloys withspecifically admeasured nickel content (Invar and Kovar alloys). Inparticular for the latter, a minimum in thermal expansion is necessarilyassociated with a decrease in yield strength.

However, it appeared that by changing the alloy composition by addingother alloy elements materials with very high yield strength R_(p0,2),reduced elastic modulus E, a high elastic range ε and outstandinghydrogen and temperature stability are provided. These “high strength(super) alloys” are found in Anglo-Saxon literature under thedesignation “thermo-span, low expansion, controlled (thermal) expansion,low coefficient of (thermal) expansion alloys”.

Chromium-nickel-steels meet these requirements only to a limited extent.

The alloy Ti-15-3-3-3 is available as a foil tape having a width of 381mm and a thickness of 1/100 mm. With smaller widths, foil thickness goesin the μm-range.

As already stated above, foils made of aluminum and its alloys aresuitable for applications in which the rather poor thermal expansioncoefficient is of minor significance. The advantage here is that, up toa minimum wall thickness, depending upon tank size and tank geometry, itis possible to manufacture seamlessly (integrally) formed liners andthen to reduce their wall thickness for example by chemical millingand/or metal-cutting processing.

The chosen foil thickness depends on one side on availability and on theother side on its processing abilities (shaping, pressure welding,handling in preparatory work for wrapping or winding, like applying ofthe wrapping or support, etc., or in the wrapping or winding processitself, respectively).

If hydrogen or cryogenic temperatures do play a role, this must beconsidered separately in selecting the liner material.

In the attached table a selection of liner materials suitable aspermeation barrier is given exemplary and without any restrictions, thatmay be used according to the invention. These materials are claimedexplicitly as liner material in conjunction with the high-pressure tanksembodied according to the invention.

The described embodiments result in a number of advantages as comparedto the plastic liner technology, in particular as relates to mass andgas tightness. Well-directed selection of the liner material alsocompatibility with extreme temperatures and media may be achieved.

The present invention under a further aspect relates to a process tomanufacture the afore stated high-pressure tanks, wherein the metal foilis mounted on a near-net-shaped core and joined by suitable methods, thesurface of the metal foil is adhesively bonded to a suitablefiber-composite material and the core is removed after hardening.

Preferred embodiments of the process according to claim 7 are given inits subclaims and are further detailed below.

A particular preference is given to a soluble core, particularlypreferably a water-soluble core. The latter may easily be dissolved fromthe tank regardless of its shape.

Manufacturing the metal foils as well as suitable joining methods areestablished state-of-the-art technologies. They have to be merelyadapted to the respective case and field of application.

Alternative methods for manufacturing thin liner layers, particularlytheir formation from liquids, gases or plasma at the inner side of theprefabricated fiber composite hull, from conditions of manufacturesuffer from insufficient tightness against high-pressure gases and/orhigh-pressure liquids consisting of very small and thus extremelypermeative atoms or molecules such as helium (He) and hydrogen (H₂). Inaddition, there are difficulties with localizing leaks related toconditions of manufacture and there is no easy repair method available.

However, starting with foils, these prior to or during their use inliner manufacture may be inspected for tightness. Owing to typical andactually unavoidable faults in foil manufacture such asentrapment/emergence of non-metal inclusions during metallurgic smeltingprocesses, imprints caused by impurities during the rolling process,etc., which cause leaks in thin foils, a critical minimum foil thicknessmust be observed. In pure terms of rolling technology, foils couldactually be much thinner, but they would then not have sufficienttightness.

The liner in a preferred embodiment is manufactured of individual partsand then joined leak-tight on the near-net-shaped core.

Manufacture of the individual parts as well as joining to liner is basedupon familiar production engineering. Shaping individual parts isaccomplished with familiar sheet-metal forming techniques.

As pertains to joining techniques, methods for bonding by materialengagement for the purpose of tightness, because of geometriccompatibility and minimization of bending stress are to prefer. Thechoice of butt joining or overlap joining technology depends upon thematerial chosen, the joining technology and geometric considerations.

For stability reasons, preferred methods are welding and brazing. In thewelding domain, fusion welding and pressure welding has to bedifferentiated. Fusion welding can be done with laser, electron beam,microplasma as well as combinations thereof. For pressure weldingrolling seam, foil seam, mash seam, glue welding, ultra sonic,diffusion, cold pressure welding, magnetic pressure welding, frictionstir welding as well as combinations thereof can be considered.

Building the liner is preferably accomplished on an interior core,because this on one side absorbs the energy required for joining theliner and then on the other side absorbs the energy for wrapping a fibercomposite.

In particular with high-pressure tanks, such an interior core afterwrapping and hardening the fiber composite and also owing to the tankdesign can frequently not be removed from the tank. The core material istherefore in this application preferably such that it upon completion ofthe tank-shaping process may be removed from the tank with the help of asolvent, e.g. water. A suitable material might for example be Aquacore®or Aquapour®. This is a high-temperature-resistant, powdery corematerial. It may be used for manufacturing complex shaping cores, e.g.by casting or injecting.

In case this core material is not suitable in conjunction with thechosen liner joining process (due to surface pressure or chemicalincompatibility), local inserts of better-suited material may beprovided on the core. For example, strips of appropriate metallicmaterials may be provided directly under the liner closing welding seam,which during the core disolving process may be pulled out through smalltank apertures.

Finally it is noted, that the embodiment and manufacture ofhigh-pressure tanks according to the invention is applicable to all tankshapes such as cylindrical, ball-shaped or cone-shaped sizes. Any othershapes, e.g. with several curvatures, are also possible. The lineritself is not a bearing element, and permeation tightness against gasesis therefore sufficient.

TABLE Tough against Tough ε = compressed at subzero ρ α R_(p0,2) ER_(p0,2)/E Material hydrogen gas temps kg/dm³ 10⁻⁶K⁻¹ MPa GPa % Aluminum7020 T6 + 0/+ 2.77 23.1 280 70 0.40 7075 T6 + 0/+ 2.80 23.4 460 70 0.65CrNi-steels 1.3974 (full + + 7.86 15.5 510 198 0.25 austenitic CrNi-steel) Thermosetting 0/+ + 7.94 17 654 200.5 0.32 CrNi-steel (A286)CuBe + + 8.25 17 1130 125 0.90 Ni-alloys Ni-Span C-902 + + 8.05 7.74 868173 0.50 Inconel 903 + + 8.25 7.2 1103 146.8 0.75 Inconel 718 0/− + 8.2013.14 523-1309 199.8 0.26-0.65 Titanium alloys Unalloyed Ti + 0/+ 4.518.6 172-482  103 0.16-0.46 Ti-5-2,5 (α) 0/+ + 4.48 9.4 720 110 0.65Ti-3-2,5 (α + β) + 0/+ 4.49 9.9 516 103 0.50 Ti-6-4 (α + β) + 0/+ 4.438.8 827 113.6 0.72 Ti-15-3-3-3 (β) − 0/− 4.76 8.5 689-965  82-1070.84-0.90 Beta 21 S (β) − 0/− 4.94 7.06 758-1277 79-105 0.95-1.2  Thestated parameters apply at room temperature ρ = density; α = thermalexpansion coefficient; R_(p0,2) = yield strength; E = elastic modulus; ε= elastic range

1. A high-pressure tank made of fiber-reinforced plastic for receivinggaseous media, wherein an inner surface of the fiber-reinforced plastictank is equipped completely or partially with a substantiallypermeation-tight foil made of metal: wherein the metal has a highelastic range and a low thermal expansion coefficient, wherein the metalis selected from the group consisting of 7020 T6, 7075 T6, 1.3974 (fullaustenitic CrNi-steel), Thermosetting CrNi-steel (A286), CuBe, Ni-SpanC-902, Inconel 903, Inconel 718, Unalloyed Ti, Ti-5-2,5 (α), Ti-3-2,5(α+β), Ti-6-4 (α+β), Ti-15-3-3-3 (β), and Beta 21 S (β), wherein thefoil has a thickness of ≦0.5 mm, and wherein the foil is contactable byan internal content of the tank.
 2. The high-pressure tank according toclaim 1, wherein the metal foil has a thickness of >1 μm and ≦10 μm. 3.The high-pressure tank according to claim 1, wherein the metal foil isimpermeable for small-atom and small-molecular gases.
 4. Thehigh-pressure tank according to claim 1, wherein the metal foil is basedupon a metal alloy resistant to compressed hydrogen gas.
 5. Thehigh-pressure tank according to claim 1, wherein the metal foil is basedupon a titanium alloy.
 6. The high-pressure tank according to claim 1,wherein the metal foil is 1.3974 (full austenitic CrNi-steel).
 7. Thehigh-pressure tank according to claim 1, wherein the metal foil isInconel
 903. 8. The high-pressure tank according to claim 1, wherein themetal foil is Inconel
 718. 9. The high-pressure tank according to claim1, wherein the metal foil is Unalloyed Ti.
 10. The high-pressure tankaccording to claim 1, wherein the metal foil is Ti-15-3-3-3 (β).
 11. Thehigh-pressure tank according to claim 1, wherein the metal foil is Beta21 S (β).
 12. The high-pressure tank according to claim 1, wherein themetal foil completely covers the inner surface of the fiber-reinforcedplastic tank.